Perfecting purification
In a study aimed at finding the most effective purifying oil for crude titanium tetrachloride, Nynas’ researchers obtained near perfection from one mineral oil and promising results from a hybrid biogenic oil.
Anna Eriksson
Marketing Manager, Naphthenics
Nynas AB
Used daily in many applications and in different forms, titanium is part of everyday life. In pure metal form and as alloys it is widely used in high performance engineering due to its corrosion resistance, light weight, and strength. However, about 95 percent of all titanium is utilised as titanium dioxide (TiO2) to produce white pigment for paints, plastics, paper, etc.
Titanium dioxide is also how it occurs in nature, where titanium is found in several different minerals. Extracting the TiO2 involves first a chlorination process that produces titanium tetrachloride TiCl4 along with chlorides of other metals present in the ore.
“Most of the metal chloride impurities can be removed from the crude TiCl4 by distillation. However, vanadium chlorides, and especially vanadium oxychloride (VOCl3), are particularly challenging, as their boiling points are very similar to that of TiCl4,” explains Anna Eriksson, Marketing Manager Naphthenics at Nynas.
As a result, the chlorination process needs to be followed by a purification step in which reagents bind to the impurities. In industry, organic reagents such as mineral or vegetable oil are often used in the purification process.
“The mechanism of organic reagents is still unclear, but the hypothesis is that the impurities and the oil form complexes of higher boiling point than the desired TiCl4, enabling them to be removed,” says Anna Eriksson.
To test the hypothesis and to assess the purifying performance of different oils, Nynas’ researchers devised a study that included several mineral oils of varying degrees of refinement, a vegetable oil, and the company’s hybrid biogenic NYTEX BIO oil.
“The results show that oils with higher solvency perform better in the purification process. They also point towards a very poor performance of severely refined oils, which supports the hypothesis that the vanadiumimpurities tend to form complexes with aromatic and alkene components in the oil. Hydrogenated paraffinic Group II oil seems not to ‘attract’ the vanadium containing impurities at all, and is therefore ineffective in this process,” says Anna Eriksson.
Before and after
Before and after treatment with NYTEX 8150. The removal of impurities in crude TiCl4 is especially important, as their presence would impact the quality of the TiO2 pigment and the hardness of the titanium metal.
Mineral and biogenic oil vs purification efficiency
The purification potency varied greatly between the different mineral oils, and there is a clear tendency of better performance with oils of a higher solvent power, in the graph indicated in yellow by low aniline points. With its excellent solvent power, NYTEX 8150 proved very effective, reaching close to 100% purification. Both biogenic oils also performed well, with NYTEX BIO offering the added advantage of not being based on food crops. The analysis of the samples was conducted at Mintek in South Africa.
Table showing some characteristics of the oils used in the study
